ORGANIC SUBSTANCES AS PLANT FOOD 



241 



of the substances absorbed from the host, and of course, greatly- 

 exhausting it in doing so. Both broom rape and dodder infest 

 only certain forms of plants; others are immune. One of the fac- 

 tors upon which depend both the germination and the penetration 

 of the broom rape into the roots of the host is a definite hydrogen- 

 ion concentration, which is close to pH 7. In a more alkaline 

 or a more acid medium germination does not take place. The 

 seeds of these parasites, es- 

 pecially of broom rape, are 

 very small and are easily 

 disseminated by wind ; besides, 

 they are capable of lying in 

 the ground for many years 

 without germinating; hence, 

 our soils always contain a 

 considerable number of seeds 

 of broom rape, and to combat 

 this parasite is possible only 

 by growing immune varieties 

 of cultivated plants. 



Still parasitism among 

 higher plants is an exception 

 rather than a rule, but among 

 fungi, which are generally 

 heterotrophic, an enormous 

 number of all kinds of para- 

 sites are found, affecting 

 higher as well as lower plants 

 and often attacking animals. 

 Bacteria are primarily animal 

 parasites, although there exists 

 a number of bacterial diseases 

 (bacterioses) of plants, es- 

 pecially in potatoes and 

 tomatoes. The study of plant diseases comprises the special 

 science of phytopathology. We do not have the opportunity 

 to cover this extensive subject here. It should be pointed out, 

 however, that similar to saprophytes, with which they are 

 united by a series of imperceptible transitions, parasites secrete 

 into the surrounding medium, the tissue of the host plant, various 



Fig. 97. 



—Dodder entwining the stem of a 

 host plant (after Brown). 



